Seismological identification of the 1998 May 28 Pakistan nuclear test

On 1998 May 28 Pakistan announced that it had conducted an underground nuclear test. Here we assess whether seismological data, recorded by the International Monitoring System (IMS) being set up to help verify the Comprehensive Test Ban Treaty (CTBT), can be used to identify the Pakistani test as a possible underground explosion. The prototype International Data Centre (pIDC) automatically determined the network-averaged body wave (m b ) and surface wave (M s ) magnitudes to be 4.9 and 3.6, respectively. One ofthe most reliable methods of identifying possible underground explosions is the m b : M s criterion. However, m b : M s is calibrated using conventional magnitudes from historical earthquakes and explosions. We calculate m b = 4.88, in the conventional way, using P waves from the Pakistani test recorded by a simulated standard short-period seismograph and read by an experienced analyst. We also analyse the three components of the surface waves from the Pakistani test to confirm that these are correctly associated, and calculate M s = 3.44. On m b : M s the Pakistani test falls between the historical Eurasian underground explosion and earthquake populations. Thus, while the source may arouse suspicion on m b : M s , its signature is typical of both explosions and deep-lithospheric Eurasian earthquakes. The vast majority of the seismic P signals from the Pakistani test, recorded at long range, are complex. However, simple P seismograms are recorded by at least three of the IMS stations. Analysis, using the relative amplitude method, of three of the simple P seismograms suggests that the source is shallow (less than 5 km). We conclude that the combination of the m b : M s signature and shallow depth are sufficient to classify the Pakistani test as a possible explosion. Under the CTBT an on-site inspection would be required to determine whether the explosion was nuclear.